1. Field of the Invention
This invention relates to steel cord belted radial pneumatic tires having improved durability comprising a rubber composition containing a novel vulcanizing agent as a coating rubber for the steel cords.
2. Description of the Prior Art
Lately, radial tires having low fuel consumption and excellent wear resistance has been widely used because of their energy- and fuel-savings properties. However, since steel cords are used as a reinforcement in these radial tires, troubles arise specifically problems in the durability of the tires due to the adhesion of rubber to steel cord arise, because the rubber composition is usually adhered to the steel cords during the vulcanization of the tire. In order to simultaneously and smoothly advance the adhesion reaction together with the vulcanization reaction, high-sulfur rubber compositions compounded with a tackifier such as cobalt salt, resorcin, hexamethylenetetramine or the like as a coating rubber for steel cords have been used. These coating rubbers can ensure adhesion to the steel cords, but are not preferable in terms of improvement of the durability of radial tire for the following reason.
That is, since the conventional coating rubber contains not less than 2 parts by weight of sulfur, the thermal aging resistance of the rubber is considerably poor, such that when the tire is run over a long period of time, polysulfide bond in the rubber changes into disulfide and monosulfide bonds. As a result, the network concentration and modulus of elasticity increase together with the running of the tire so as to deteriorate fatigue properties such as the breaking strength, the breaking extension and the like. Thus, cracks are produced and grow from a stress concentrated portion at the end portions of the steel cords to cause a so-called belt end separation. In order to improve this drawback, an attempt to improve the thermal aging resistance has been made by adding a vulcanization accelerator in an amount substantially equal to the sulfur compounding amount as in a so-called effective vulcanization. However, this attempt is no practical because the adhesion between the rubber and steel cord considerably lowers when using coating rubber for steel cords.
Furthermore, the blooming of the sulfur during the manufacture of the tires comes into question with coating rubber having a higher sulfur content. Since sulfur is poorly in the soluble rubber, when steel cords covered with a sulfur-containing rubber composition or so-called treatment cords are temporarily stored over about a week during the tire manufacturing step, a phenomenon of dissolving out the sulfur from the rubber arises and the sulfur crystallizes on the surface of the rubber. This is the so-called blooming phenomenon. In general, the tire is manufactured by laminating the treatment cords one upon the other at the tire building step, so that if sulfur is bloomed on the rubber surface, the tackiness between mutual rubbers considerably lowers and the lamination is made by insufficient adhesion. As a result, when the tire is used at a high temperature during the runing, the adhesion peeling or so-called separation failure is produced between the laminated steel cord reinforcing layers due to the insufficient durability of the coating rubber for the steel cords.
If the adhesion of the coating rubber to the steel cord as described above is improved, the thermal aging resistance of the coating rubber and the tackiness between the mutual coating rubbers are conversely degraded. Therefore, it is necessary to simultaneously improve such two contrary performances in order to improve the durability of the steel cord belted radial tire.